An apparatus includes an extractor, a first subtractor, a second subtractor, a third subtractor, an offset generator and an adder. The extractor receives the color pixel and to provides a first, a second, and a third image value, wherein the first, the second, and the third image values are associated with the brightness of the color pixel. The first subtractor calculates a first offset between the first image value and a first expected image value. The second subtractor calculates a first difference between the first and the second image values. The third subtractor calculates a second difference between the first and the third image values. The offset generator defines a linear relation based on the first offset and the first image value, and generates a second and a third offset. The adder receives the second offset, the third offset, the second image value and the third image value to generate a second and a third expected image value.